The present invention generally relates to methods and apparatus for the treatment of waste and more particularly to methods and apparatus for the treatment of waste using arc plasma-joule heated melter systems.
The disposal of municipal solid waste (MSW) and other waste has become a major issue over the past few decades due to space limitations for landfills and problems associated with siting new incinerators. In addition, increased environmental awareness has resulted in a major concern of many large metropolitan areas and to the country as a whole to ensure that the disposal of solid waste is properly handled. See e.g., USA EPA, The Solid Waste Dilemma: An Agenda for Action, EPA/530-SW-89-019, Washington, D.C. (1989).
Attempts have been made to reduce the volume and recover the energy content of MSW through incineration and cogeneration. The standard waste-to-energy incinerator will process the solid combustible fraction of the waste stream, produce steam to drive a steam turbine, and as a result of the combustion process produce a waste ash material. Typically, the ash is buried in a municipal landfill. Current trends and recent rulings, however, may require such material to be shipped to landfills permitted for hazardous waste. This will substantially increase ash disposal costs. Moreover, there is increased public concern about gaseous emissions from landfills and the possibility of contamination of groundwater. Another disadvantage associated with incinerator systems is the production of large quantities of gaseous emissions resulting in the need for costly air pollution control systems in an attempt to decrease emission levels to comply with requirements imposed by regulatory agencies.
In order to overcome the shortcomings associated with incinerator systems, attempts have been made in the prior art to utilize arc plasma torches to destroy toxic wastes. The use of arc plasma torches provides an advantage over traditional incinerator or combustion processes under certain operating conditions because the volume of gaseous products formed from the plasma arc torch may be significantly less than the volume produced during typical incineration or combustion, fewer toxic materials are in the gaseous products, and under some circumstances the waste material can be glassified.
For example, U.S. Pat. No. 5,280,757 to Carter et al. discloses the use of a plasma arc torch in a reactor vessel to gasify municipal solid waste. A product having a medium quality gas and a slag with a lower toxic element leachability is produced thereby.
U.S. Pat. No. 4,644,877 to Barton et al. relates to pyrolytic destruction of polychlorinated biphenyls (PCBs) using a plasma arc torch. Waste materials are atomized and ionized by a plasma arc torch and are then cooled and recombined into gas and particulate matter in a reaction chamber. U.S. Pat. No. 4,431,612 to Bell et al. discusses a hollow graphite electrode transfer arc plasma furnace for treatment of hazardous wastes such as PCBs.
A process for remediation of lead-contaminated soil and waste battery material is disclosed in U.S. Pat. No. 5,284,503 to Bitler et al. A vitrified slag is formed from the soil. Combustible gas and volatized lead, which are formed from the waste battery casings, are preferably transferred to and used as a fuel for a conventional smelting furnace.
The systems proposed by Barton et al, Bell et al, Carter et al, and Bitler et al have significant disadvantages. For example, such disadvantages include insufficient heating, mixing and residence time to ensure high quality, nonleachable glass production for a wide range of waste feeds. Additionally, hearth size and feeder design are significantly limited since furnace walls must be relatively close to the arc plasma which is the only heat source. High thermal stress on the walls of the furnace often occurs as a result of the limitation on the hearth size.
Prior art arc plasma furnaces with metal electrodes further may be limited by short electrode lifetime when used at higher DC current. Therefore, to achieve higher power output, the arc potential must be raised by lengthening the arc. This results in radiative thermal losses to the furnace side walls and leads to metal electrode (torch) ineffectiveness. In addition, there are often difficulties associated with prior art transfer arc plasmas in start-up and restarting of such arc plasma systems when cold, nonelectrically conducting material is being processed.
Another disadvantage associated with prior art systems is the inefficient use of combustible gases produced during conversion of waste materials. For example, combustion of gases often does not result in a high conversion rate and is thus inefficient. In addition, the combustion of such gases frequently results in the emission of pollutants such as oxides of nitrogen (NOx) in amounts that render the process environmentally unattractive.
Thus, while such prior art attempts have been useful, there remains a need in the art for a robust, easy to operate waste conversion system which minimizes hazardous gaseous emissions and which maximizes conversion of a wide range of solid waste into useful energy and produces a product stream which is in a safe, stable form for commercial use or which does not require special hazardous waste considerations for disposal.
It would therefore be desirable to provide robust, user friendly and highly flexible methods and apparatus for processing and converting a wide range of waste materials into useful energy and stable products while minimizing hazardous gaseous emissions, thereby overcoming the shortcomings associated with the prior art.
The present invention provides methods and apparatus for increased conversion of solid waste materials such as municipal and industrial waste to useful energy with greatly reduced air pollution. The present invention additionally provides methods and apparatus for converting a wide range of waste materials to useful commercial products or to safe, stable products which are suitable for disposal. For example, the systems of the present invention are capable of processing municipal solid waste (MSW), industrial waste or other waste forms into stable nonleachable products (e.g. crystalline and noncrystalline products) which are suitable for use commercially or which can be disposed of without risk to the environment. The systems also minimize air emissions and maximize production of a useful gas product for the production of electricity. The present invention further provides compact waste-to-energy processing systems that have the advantage of complete or substantially complete conversion of waste materials into a useful gas and a product stream at a single location.
The methods and apparatus of the present invention for converting waste materials employ combinations of an independently controllable arc plasma or plasmas and a joule heated melter as an integrated system. In a preferred embodiment of the invention, a fully integrated joule heated melter and arc plasma or plasmas are utilized for the conversion of waste materials. The joule heated melter and arc plasma or plasmas are configured for simultaneous operation with a common molten pool without detrimental interaction of the power supplies and with independent control of the power to each of the joule heated melter portion of the system and the arc plasma portion of the system.
As stated above, the products formed from the conversion process of the invention are useful commercial products or stable products. suitable for disposal. In addition, the present invention allows the use of fast pyrolysis, thereby providing a high purity gas suitable for use in combustion and non-combustion processes. For example, the gases produced in accordance with the invention can be used to generate electricity using a small, highly efficient gas turbine or internal combustion engine. In some embodiments, the waste conversion units of the invention can be self-powered or can provide a given level of electricity for outside use. This may be accomplished by utilizing an auxiliary fuel, such as natural gas, diesel or some other fuel, in varying amounts in the gas turbine or internal combustion engine.
In alternative embodiments of the invention, environmentally attractive low emission internal combustion engine-generator systems (or gas turbine systems) for waste treatment units are provided to greatly improve efficiency and pollution reduction. This is accomplished by utilizing multi-fuel (e.g. hydrogen-rich gas, natural gas, diesel oil) operation of the spark ignition engine at ultra lean ratios of fuel to air. Ultra lean operation is made possible by the fast flame front characteristic of the hydrogen in the hydrogen-rich gas produced by the waste treatment unit. In addition, very high compression ratios could be used in the internal combustion engine. Variable fuel operation is made possible by control and fuel processing systems that ensure that ultra lean, high compression ratio engine requirements for smooth burn ignition and lack of knock are met under continually varying fuel conditions.
It is expected that the high efficiency, low emission internal combustion engine-generator systems could increase the efficiency of conversion of gaseous fuel to electricity by up to approximately 40% (for example from 30 to 42%). It is also expected that by operating at ultra lean conditions such systems can reduce NOx emission by factors of more than ten relative to standard internal combustion engine-generator systems. An additional objective of the invention is to provide an option to utilize such systems to reduce carbon monoxide and hydrocarbon emissions by factors of more than ten by using highly robust and simple oxidation catalysts. For example, the invention provides environmentally attractive systems designed to utilize spark ignition internal combustion engines at ultra lean ratios of fuel to air (in the range of about 0.4-0.7 relative to stoichiometric ratios) and at very high compression ratios, e.g. r in the range from about 12 to 15, or for operating turbines at ultra lean ratios of fuel to air so as to significantly reduce levels of NOx production.
In another embodiment of the invention, the off-gas from the waste conversion unit can be employed in a non-combustion process. This can be accomplished by integrating the waste conversion units with fuel cell systems for the efficient and environmentally favorable production of electrical energy from waste processed in the waste conversion unit. For example, a molten carbonate fuel cell (MCFC) can be used in conjunction with the waste conversion units of the present invention to produce electricity from the waste conversion unit off-gases in a non-combustion process provided that the off-gases are sufficiently clean for use with the fuel cell. This may be desirable for example when composition of the waste is such that the furnace off gas produced by the waste will be compatible with the fuel cell, thereby permitting the DC power output of the fuel cell to be converted to three phase AC power for sale to a utility company or for use in powering the waste conversion unit.
The combination of the arc plasma furnace and the joule heated melter as an integrated system with gas turbine, internal combustion engine or fuel cell generating equipment provides waste treatment and power production facilities which are capable of being deployed in modular units and which can be easily scaled to handle large volumes of municipal solid waste.
The primary processing unit preferably includes a DC or AC electrode arc plasma or plasmas for heating waste material and which also has joule heating capability for the melt pool. In a preferred embodiment, the electrode arc or arcs is a DC electrode arc or arcs with electrodes formed of graphite. The use of a DC or AC arc electrode(s) in combination with the appropriate electrical circuit ensures simultaneous independent control of the arc plasma(s) and the joule heated melter systems. The primary mode of operation of the arc plasma and joule heated melter is pyrolysis (i.e. oxygen starved operation). In a preferred embodiment, the system is operated such that fast pyrolysis occurs, thereby producing a gas with higher purity as compared with other methods of pyrolysis.
Preferably, the arc plasma and joule heated melter components are fully integrated with a common molten pool such that the system is capable of simultaneous independently controllable, i.e. tunable, operation of these components. The arc plasma(s) occurs between a graphite electrode or electrodes and the molten material. It should be appreciated, however, that other metallics elements such as tungsten or the like may be utilized as the electrode material rather than graphite.
The tunable fully integrated systems of the present invention employ electrical and mechanical design features to maximize flexibility and effectiveness. In this manner, high processing rates for vitrification of a large variety of materials into high quality, stable, non-leachable glass and reduced volume requirements due to the integrated system can be expected. The arc plasma(s) provides the necessary radiant surface heating for processing feed material in a highly efficient manner and at significantly higher rates than other technologies. The joule heated melter provides deep volume heating and is capable of maintaining a constant temperature throughout the melt pool with uniform mixing characteristics, thereby resulting in a high quality, homogenous glass product.
Simultaneous independently controllable operation of the arc plasma(s) and joule heated melter is provided by predetermined arc melter configurations and electrical circuits. While not meant to be limiting, the arc plasma preferably is operated by a DC arc or arcs and the joule heated melter is operated by AC power. The DC arc(s) and AC powered joule heated melter arrangement ensures the ability to independently control and operate each component. In alternative embodiments, however, both the arc(s) and the joule heated melter portions can be operated with AC power while maintaining the ability to independently control and operate each component or portion.
The present invention provides DC and AC arc circuits which allow independent arc voltage and current control. These circuits can be designed for operation with one arc electrode or, in the alternative, with a plurality of arc electrodes. These circuits can also be designed for switching between AC and DC, as such power is desired. The present invention also provides joule heating circuits that can be operated simultaneously and independently of the arc plasma or plasmas.
The use of the melter in combination with the arc plasma(s) provides more uniform heating than prior art techniques. In addition, utilizing deep volume heating provided by the joule heated glass melter facilitates ease of operation. It also provides the constant heat source necessary to maintain sufficient electrical conductivity in the waste material for rapid restart of the arc plasma which uses or can use an electrical conduction path through the waste material. Additionally, the fully integrated system allows the furnace walls to be further from the arc plasma(s) since there is an additional heat source provided. The increase in wall distance from the arc plasma increases feed options and reduces thermal stress on the furnace lining. Consequently, thermally sensitive, highly durable, long-life refractory linings can be employed. The present invention further allows the use of electrodes having a long life and a very wide range of arc plasma and joule heater power levels.
The independent control of the arc plasma and the joule heated melter power provides a continuously tunable mix of surface and deep volume heating, which can be optimized for different phases of operation. For example, additional heating may be desired or required for pouring glass or maintaining the glass pool temperature while additional surface heating may be necessary during the initiation of waste feed. In addition, different mixes of surface and volume heating are appropriate for different waste streams. The ratio of surface to deep volume heating may be less for municipal waste, for example, than for industrial waste containing large amounts of metals and high temperature materials. The control of power to each of the arc plasma(s) and joule heated melter portions can be adjusted (manually or automatically) during processing and operation to account for such different phases of operation.
The high quality, vitrified products produced in accordance with the present invention may be used in a variety of applications. For example, the vitrified products may be crushed and incorporated into asphalt for use in roads and the like. Alternatively, the vitrified products may be utilized to replace cinder in cinder or building blocks, thereby minimizing absorption of water within the block. Further, the vitrified products may be solidified to a final form which exhibits substantial volume reduction over prior art vitrification products. The products formed in accordance with the present invention can also be of a crystalline structure or a combination of crystalline and non-crystalline structures. The solidified forms are suitable for disposal without health risks or risks to the environment.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner of modifying the invention as will be described. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the following Detailed Description of the Preferred Embodiments.